One trend in mobile communication networks is that they are becoming more and more heterogeneous. This means that the base stations and the cells they provide vary considerably. There may here be macro cells covering large areas combined with much smaller pico cells covering hot spots. A macro base station does typically transmit with much higher power than a pico cell and hence provides much larger cells. In addition to this it is possible that the networks also includes femto cells, where the base stations that provide these cells are even smaller and transmit with even lower power levels. These cells are often provided domestically or in households.
As a mobile station is moving in such a network, it is common to hand it over from one cell to another. In order to be able to perform handover, the cells have to be possible to identify. For this reason the cells are provided with cell identifiers. The identifiers used for handover are then often shared by several cells, i.e. not unique. However, the re-use of cell identifiers has traditionally been such that the cells that share the same identifier are provided at long distances from each other and therefore the same identifier should not appear in cells within the same neighbourhood. This is typically done by the operator in network planning.
Now with the introduction of pico and femto cells, this may not be possible any more. There may be so many cells that several pico cells may use the same cell identifier in the vicinity of a macro cell. There may here be even more femto cells in a neighbourhood.
Furthermore, a network operator may also have no real control of where a femto cell is placed. Therefore also several of these may have the same non-unique cell identifier.
Since it is possible for a target cell, i.e. a cell to which a mobile station may need to be handed over, to share a cell identifier with other cells, there may occur problems when performing handover from a source cell, i.e. a cell with which a mobile station is actively communicating.
One way to handle this is described in WO 2008/113373, where a forced handover command is used by the source base station to command a mobile station to perform handover to a target base station for which a cell global identifier is not known in the source base station. The source base station also provides information about its own cell global identifier to the mobile station that forwards the information to the target base station. Target and source base station may then communicate for effecting handover.
Another way to address this is to be found in 3GPP TS 36.300 in a procedure called “inbound mobility to E-UTRAN CSG cells”. Here principles similar to Automatic Neighbour Relation (ANR) are applied for letting the mobile station provide more information to the source cell to uniquely identify the target cell.
This type of activity functions well. However, there is still room for improvement when performing handover to a cell with non-unique cell identifier.